Many home and commercial fire alarm systems employ a design which is dependent upon remote sensors. Since the most practical method of transmitting information over any distance is through electrical means, most alarms of this type rely on batteries, house power, or a combination of the two. Most electronic sensing systems utilize a low voltage sensing circuit to provide information to a central control center. In very large installations, such as office buildings, motels, hospitals, and industrial complexes, an additional independent power source must be utilized if an alarm is to be sounded in the immediate vicinity of the triggered sensor. Consequently, the advantage of having a central control center is offset by the added expense of installing independently powered alarm systems to provide local as well as general alarms. Unfortunately, remote sensor electronic alarms are prone to a number of disabling problems. Battery powered units must be checked frequently and provide maximum security only immediately after recharge or replacement. Systems operating on house power are sensitive to power interruptions and units which incorporate both means are costly and seldom work interchangeably on the different power modes.
The other common type of alarm relies upon remote independent self-powered units which are usually triggered in the presence of excessive heat. Although these units may be powered by house power or battery, since there is no need to transmit information over great distances, physical means may be employed. One such method employs a freon containing cylinder connected to a freon horn through a heat sensing trigger. This type of system is fully described in U.S. Pat. No. 3,667,419. The best of these units may last up to 30 years without recharging. They have the advantage of complete independence from house power interruption and battery deterioration. Once they are triggered they provide a piercing local alarm which may continue until the area is evacuated.
Remote independently powered alarms are presently limited to applications where it is necessary to provide no more than a local alarm. In a large installation, such as a hotel, it is difficult for the proprietors to hear and establish the exact location of a triggered alarm. Furthermore, independently powered alarms are usually unable to perform tasks such as sounding a general alarm, or directly notifying the police and fire departments. As a result, these alarms have been restricted to installations where a piercing local alarm provides adequate protection.